Method of forming high temperature superconducting Josephson junction

ABSTRACT

A method of forming a novel high temperature superconducting Josephson junction which is capable of achieving a formation of a Josephson junction having high characteristics conveniently and quickly without necessitating costly micromachining facilities. Two high temperature superconducting whisker crystals are crossed with each other on a substrate and subjected to thermal treatment to form a Josephson junction between the two high temperature superconducting whisker crystals.

This application is a Divisional application of Ser. No. 09/932,941,filed Aug. 21, 2001 now U.S. Pat. No. 6,682,621.

BACKGROUND OF THE INVENTION

The present invention relates to methods of forming high temperaturesuperconducting Josephson junctions, more particularly, to a method offorming a high temperature superconducting Josephson junction whichachieves a formation of a Josephson junction having high characteristicsconveniently and quickly without necessitating costly micromachiningfacilities.

A high temperature superconducting Josephson device has gained a highattention and has been required to be commercialized as a new electronicdevice which realizes, for example, an ultrahigh frequency generatingdevice capable of generating frequencies in a range of terahertz ormore, an ultrahigh frequency detecting device capable of detectingfrequencies in a range of terahertz or more, a SQUID (superconductingquantum interference device) magnetometric sensor and, further, adigital device carrying a mass information processing system.

On the other hand, when the Josephson junction is formed using a singlecrystal or a thin film, the micromachining facilities which are used forproducing a semiconductor device such as a convergence ion beam etchingdevice and a photolithography device has ordinarily been employed.There, however, is a problem in using the micromachining facilities forproducing such devices that, for example, not only a tremendously highcost, but also a complexity of production process and an extended periodof production time are necessary. Further, it is difficult to conduct astudy associated with the Josephson junction at a research institutewithout having the micromachining facilities whereupon, since themicromachining facilities are extremely highly priced, it is a presentsituation that a number of researchers who are related with theJosephson junction is limited.

Further, it is considered essential to achieve downsizing the Josephsondevice from a viewpoint of, for example, prevention of malfunction to becaused by heat generation, reduction of power consumption, enhancementof integration and enhancement of characteristics.

SUMMARY OF THE INVENTION

The present invention has been accomplished under these circumstancesand has an object to provide a method of forming a novel hightemperature superconducting Josephson junction which achieves aformation of a Josephson junction having high characteristicsconveniently and quickly without necessitating a costly micromachiningfacilities.

In order to attain the above-described object, the present inventionprovides, as a first aspect, a method of forming a high temperaturesuperconducting Josephson junction comprising the steps of:

crossing high temperature superconducting whisker crystals with eachother;

subjecting the thus crossed high temperature superconducting whiskercrystals to thermal treatment; and

forming a Josephson junction in a combined portion of the hightemperature superconducting whisker crystals or in a neighborhood of thecombined portion.

As a second aspect, the present invention provides a method of forming ahigh temperature superconducting Josephson junction comprising the stepsof:

crossing linear high temperature superconducting thin filmsmask-processed into an elongate linear shape with each other on asubstrate;

subjecting the thus crossed linear high temperature superconducting thinfilms mask-processed into an elongate linear shape to thermal treatment;and

forming a Josephson junction between the linear high temperaturesuperconducting thin films.

As a third aspect, the present invention provides a method of forming ahigh temperature superconducting Josephson junction comprising the stepsof:

crossing a linear high temperature superconducting thin filmmask-processed into an elongate linear shape and a high temperaturesuperconducting whisker crystal;

subjecting the thus crossed linear high temperature superconducting thinfilm mask-processed into an elongate linear shape and high temperaturesuperconducting whisker crystal to thermal treatment; and

forming a Josephson junction between the linear high temperaturesuperconducting thin film and the high temperature superconductingwhisker crystal.

As a fourth aspect, the present invention provides the method of formingthe high temperature superconducting Josephson junction of theabove-described first aspect in which one or both of the hightemperature superconducting whisker crystals are substituted by a linearhigh temperature superconducting monocrystal processed into an elongatelinear shape.

As a fifth aspect, the present invention provides the method of formingthe high temperature superconducting Josephson junction of theabove-described third aspect in which the high temperaturesuperconducting whisker crystal is substituted by a linear hightemperature superconducting monocrystal processed into an elongatelinear shape.

As a sixth aspect, the present invention provides the method of formingthe high temperature superconducting Josephson junction of any one ofthe foregoing aspects in which the high temperature superconductingwhisker crystal, the linear high temperature superconducting thin filmor the linear high temperature superconducting monocrystal which is toform a Josephson junction is a compound containing at least one phase ofBismuth 2212 phase, Bismuth 2201 phase and Bismuth 2223 phase.

As a seventh aspect, the present invention provides the method offorming the high temperature superconducting Josephson junction of anyone of the foregoing aspects in which the high temperaturesuperconducting whisker crystal, the linear high temperaturesuperconducting thin film or the linear high temperature superconductingmonocrystal which is to form a Josephson junction turns mutual thec-face up, and forming a Josephson junction.

As an eighth aspect, the present invention provides the method offorming the high temperature superconducting Josephson junction of anyone of the foregoing aspects in which the high temperaturesuperconducting whisker crystal, the linear high temperaturesuperconducting thin film or the linear high temperature superconductingmonocrystal which is to form a Josephson junction are adjusted at 45 to90 degrees in the cross angl, and forming a Josephson junction.

Further, as a ninth aspect, the present invention provides the method offorming the high temperature superconducting Josephson junction of anyone of the foregoing aspects in which the Josephson junction to beformed is an intrinsic Josephson junction.

Furthermore, as a tenth aspect, the present invention also provides ahigh temperature superconducting Josephson device provided with aJosephson junction to be formed by the method of forming the hightemperature superconducting Josephson junction of any one of theforegoing aspects in which the high temperature superconductingJosephson device using other portions than a junction portion of a hightemperature superconducting whisker crystal, a linear high temperaturesuperconducting thin film or a linear high temperature superconductingmonocrystal imparted with the Josephson junction as a wire material fora wire connection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph showing a Josephson junction formed in anembodiment according to the present invention;

FIG. 2 is a graph showing a result of measurements of current-voltagecharacteristics of a Josephson junction formed in an embodimentaccording to the present invention; and

FIG. 3 is another graph showing a result of measurements of acurrent-voltage characteristics of a Josephson junction formed in anembodiment according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention has characteristics as described above;embodiments thereof will be described below.

In a method of forming a high temperature superconducting Josephsonjunction according to the present invention, a Josephson junction havingan extremely minute junction area is achieved by making use of acharactristic that a high temperature superconducting whisker crystal isin a needle like elongate shape. Specifically, for example, two hightemperature superconducting whisker crystals are placed on a substratesuch that they are crossed with each other and, then, subjected tothermal treatment to form a Josephson junction in a junction portion orin a neighborhood thereof. Since the junction area is extremely minute,it is not necessary to perform a special micromachining. About thetemperature of thermal treatment, making a maximum into the meltingpoint of a sample to use and making a minimum into about 500 degrees C.for example is illustrated as a general standard. More preferably, it isdesirable to consider as 700 degrees C. or more. About a thermaltreatment atmosphere, it is desirable that oxygen is contained at least.

In the method of forming the high temperature superconducting Josephsonjunction according to the present invention, a linear high temperaturesuperconducting thin film mask-processed into an elongate linear shapemay be used instead of using the high temperature superconductingwhisker crystal. On this occasion, for example, two linear hightemperature superconducting thin films are placed on the substrate suchthat they are crossed with each other and then subjected to thermaltreatment thereby forming a Josephson junction between the two linearhigh temperature superconducting thin layers. Further, it is permissiblethat the high temperature superconducting whisker crystal and the linearhigh temperature superconducting thin film are placed each by one on thesubstrate such that they are crossed with each other and, then,subjected to thermal treatment to form the Josephson junctiontherebetween.

Furthermore, in the present invention, the above-described whisker maybe substituted by the linear high temperature superconductingmonocrystal processed into an elongate linear shape. Although it isknown for the high temperature superconductor that a whisker will growup to be only a bismuth system, a Josephson junction can be formed froma bismuth system high temperature superconductor or the other hightemperature superconductor by cutting down a single crystal long andslender and using it.

In the method of forming the high temperature superconducting Josephsonjunction, the high temperature superconducting whisker crystal, thelinear high temperature superconducting thin film or the linear hightemperature superconducting monocrystal which is to form a Josephsonjunction may be various kinds of high temperature superconductors,preferably, is a compound which mainly contains either Bismuth 2212phase, Bismuth 2201 phase and Bismuth 2223 phase. Moreover, it may be acompound containing at least two or more sorts of mixed phases ofBismuth 2212 phase, Bismuth 2201 phase and Bismuth 2223 phase.

Examples of Josephson junctions to be formed by the method of formingthe high temperature superconducting Josephson junction according to thepresent invention include an intrinsic Josephson junction. Ordinarily,it is extremely difficult to produce the intrinsic Josephson junctionhaving a small number of junction layers; however, the method of formingthe high temperature superconducting Josephson junction according to thepresent invention is capable of forming the intrinsic Josephson junctionconstituted with one layer or a small number of junction layers.

In the method of forming the high temperature superconducting Josephsonjunction, the high temperature superconducting whisker crystal, thelinear high temperature superconducting thin film or the linear hightemperature superconducting monocrystal which is to form a Josephsonjunction can form a Josephson junction in arbitrary crystal facesmutually.

For example, a Josephson junction of the a-face and the a-face, aJosephson junction in the b-face and the b-face, etc. are possible. AndThe optimum combination of a crystal face is the case where turns mutualthe c-face up, and forming a Josephson junction. That is, it is joiningthe c-face. Moreover, the Josephson characteristic is artificiallycontrollable by changing across angle of these high-temperaturesuperconductors. Specifically, Josephson current serves as the maximum,when for example, across angle is about 90 degrees, and it serves as theminimum at about 45 degrees. Therefore, in case Josephson junction isapplied as an element, it is enabled to control artificially the timing(current value) of switching for every element.

Further, in the high temperature superconducting Josephson device whichhas been provided with the Josephson junction formed by the method offorming the high temperature superconducting Josephson junctionaccording to the present invention, other portions than a junctionportion of the high temperature superconducting whisker crystal, thelinear high temperature superconducting thin film or the linear hightemperature superconducting monocrystal which has been imparted with aJosephson junction are used as a wire material for a wire connectionterminal whereby a wire connection in a superconducting state can berealized to materialize prevention of a malfunction caused by heatgeneration in the Josephson device and reduction of power consumption.

The present invention has the characteristics as described above andwill further be described in detail by embodiments below.

EXAMPLE

A Josephson junction was formed by a method of forming a hightemperature superconducting Josephson junction according to the presentinvention. As a sample, two Bismuth 2212 high temperaturesuperconducting whiskers were placed on an MgO substrate such that theyare crossed with each other to be in a cross shape. Subsequently, suchtwo high temperature superconducting whisters were placed in a furnacetogether with the substrate and subjected to thermal treatment under acondition that an oxygen atmosphere was controlled. As sinteringconditions, a temperature was 850° C., an oxygen partial pressure was70% and a period of time of sintering was 30 minutes.

FIG. 1 shows a photograph of a sample after subjected to thermaltreatment. An end portion of the thus thermally treated sample was thenprovided with a current-voltage terminal. Current-voltagecharacteristics of a Josephson junction portion of the resultant samplewere measured. FIG. 2 shows a result of the thus obtained measurementsFurther, a Bismuth 2212 high temperature superconducting whisker and aBismuth 2212 linear high temperature superconducting thin film wereplaced on an MgO substrate such that they were crossed with each otherto be in a cross shape and subjected to thermal treatment undersintering conditions described above to obtain a sample. FIG. 3 shows aresult of measurements of the current-voltage characteristics of theJosephson junction of the thus obtained sample. It is found from FIGS. 2and 3 that advantageous superconducting characteristics have beenobtained in each of the above-obtained samples.

As described in detail above, according to the present invention,provided is a method of forming a novel high temperature superconductingJosephson junction to achieve a formation of a Josephson junction havinghigh characteristics conveniently and quickly without necessitatingcostly micromachining facilities. The present invention contributes toactivation of research of a superconducting device and, further, withregard to the superconducting device, has a variety of technical effectssuch as reduction of production cost, simplification of productionfacilities and reduction of production time period. The Josephson deviceproduced by the present invention is imparted with excellentfunctionality and characteristics; hence, capability of applicationsthereof to various fields such as communication technology, sensortechnology and information processing technology can be considered andare strongly required to be commercialized.

1. A high temperature superconducting Josephson device provided with aJosephson junction, the high temperature superconducting Josephsondevice using other portions than a junction portion of a hightemperature superconducting whisker crystal imparted with the Josephsonjunction as a wire material for a wire connection, the Josephsonjunction being formed by a method comprising: crossing high temperaturesuperconducting whisker crystals with each other; subjecting the thuscrossed high temperature superconducting whisker crystals to thermaltreatment; and forming a Josephson junction in a combined portion of thehigh temperature superconducting whisker crystals or in a neighborhoodof said combined portion.
 2. A high temperature superconductingJosephson device provided with a Josephson junction, the hightemperature superconducting Josephson device using other portions than ajunction portion of a linear high temperature superconducting thin filmimparted with the Josephson junction as a wire material for a wireconnection, the Josephson junction being formed by a method comprising:crossing linear high temperature superconducting thin filmsmask-processed into an elongate linear shape with each other; subjectingthe thus crossed linear high temperature superconducting thin filmsmask-processed into an elongate linear shape to thermal treatment; andforming a Josephson junction between the linear high temperaturesuperconducting thin films.
 3. A high temperature superconductingJosephson device provided with a Josephson junction, the hightemperature superconducting Josephson device using other portions than ajunction portion of a high temperature superconducting whisker crystalor a linear high temperature superconducting thin film imparted with theJosephson junction as a wire material for a wire connection, theJosephson junction being formed by a method comprising: crossing alinear high temperature superconducting thin film mask-processed into anelongate linear shape and a high temperature superconducting whiskercrystal; subjecting the thus crossed linear high temperaturesuperconducting thin film mask-processed into an elongate linear shapeand high temperature superconducting whisker crystal to thermaltreatment; and forming a Josephson junction between the linear hightemperature superconducting thin film and the high temperaturesuperconducting whisker crystal.
 4. A high temperature superconductingJosephson device provided with a Josephson junction, the hightemperature superconducting Josephson device using other portions than ajunction portion of a high temperature superconducting whisker crystalor a linear high temperature superconducting monocrystal imparted withthe Josephson junction as a wire material for a wire connection, theJosephson junction being formed by a method comprising: crossing alinear high temperature superconducting monocrystal processed into anelongate linear shape with another linear high temperaturesuperconducting monocrystal processed into an elongate linear shape or ahigh temperature superconducting whisker crystal; subjecting the thuscrossed crystals to thermal treatment; and forming a Josephson junctionin a combined portion of the crystals or in a neighborhood of saidcombined portion.
 5. A high temperature superconducting Josephson deviceprovided with a Josephson junction, the high temperature superconductingJosephson device using other portions than a junction portion of alinear high temperature superconducting thin film or a linear hightemperature superconducting monocrystal imparted with the Josephsonjunction as a wire material for a wire connection, the Josephsonjunction being formed by a method comprising: crossing a linear hightemperature superconducting thin film mask-processed into an elongatelinear shape and a linear high temperature superconducting monocrystalprocessed into an elongate linear shape; subjecting the thus crossedthin film and monocrystal to thermal treatment; and forming a Josephsonjunction between the thin film and the whisker crystal.
 6. The hightemperature superconducting Josephson device as set forth in claim 1,wherein the high temperature superconducting whisker crystals, which areto form a Josephson junction are a compound containing at least onephase of Bismuth 2212 phase, Bismuth 2201 phase and Bismuth 2223 phase.